The present system and method is generally applicable to communication systems for receiving signals in a high noise environment, and specifically of systems which monitor radio frequencies to determine the presence of a transmitted waveform. Detection of transmitted waveforms may be necessary in order to prevent transmission collisions or may be desired for monitoring or surveillance systems whose goal is to detect and exploit received signals. Although described with respect to the HF radio propagation band, the present system and method is equally applicable to all communication systems and radio frequency bands.
Collision avoidance is an important consideration in communication systems due to the limited bandwidth available. Collision avoidance systems allow an increase in the throughput of the communication system by reducing the number of collisions and thus reducing the need to retransmit signals that were not successfully received. Many communication systems or networks, both wired (e.g., Ethernet) and wireless (e.g., HF, VHF, UHF radio) utilize a form of Carrier Sense Multiple Access (CSMA) to determine whether a frequency or radio channel is being used by another station or stations prior to transmission. Systems utilizing CSMA typically detect the signal energy on the desired channel and classify the channel as in use if a predetermined threshold is exceeded. However, collision avoidance systems which rely on the detection of signal energy, generally require an environment having a positive signal-to-noise (SNR) and do not work well in the noise and interference rich environment of radio communications where negative SNRs are common.
Signal detection of standard HF waveforms is further hampered by the use of heavily filtered Phase Shift Keyed (PSK) and Quadrature-Amplitude Modulated (QAM) waveforms. These waveforms are extremely noise-like in nature and difficult to distinguish from background noise and interference.
Conventional prior art waveform detection systems can not detect or classify a modulated waveform without first demodulating the received signal. These systems are computationally intense because the standard demodulation of the modem signal from the sub-carrier requires the demodulators to perform many functions including (a) waveform acquisition, (b) adaptive equalization, (c) forward error correction (FEC), (d) decoding, and (e) phase, time, and frequency offset tracking.
Additionally, because the received signal needs to be demodulated, the beginning of the transmission has to be received and recognized by the detection system to ensure proper demodulation.
Many of the waveforms defined in U.S. Military and NATO (STANAG) HF Standards utilize interspersed blocks of known data, commonly referred to as training sequences, in their transmissions. These blocks of known data can aid demodulators in training adaptive equalizers to track variations of the radio propagation channel caused by fading and multi-path conditions.
In addition to prevent collisions between separate transmissions, the detection and classification of waveforms is also an important consideration for surveillance systems which are designed to locate and exploit transmitted waveforms.
The present system and method advantageously exploits the transmitted training sequence of a signal to detect the presence of a waveform without requiring the full demodulation process that would be necessary to regenerate the transmitted data. Further the present system and method can classify the type of waveform detected.
Accordingly, it is an object of the present invention to provide a novel method and system for detecting standard modem transmissions without requiring demodulation of the signals.
It is another object of the present invention to provide a novel system and method to detect the transmission of a waveform in the presence of a frequency offset without compensating for the frequency offset.
It is a yet another object of the present invention to provide a novel system and method to detect the presence of a transmitted waveform in a very high noise and interference environment.
It is still another object of the present invention to provide a novel system and method to detect the presence of a transmitted waveform utilizing a fairly short observation period (approximately 2400 transmitted symbols).
It is yet still another object of the present invention to provide a novel system and method to classify the type of received modulated waveform without requiring demodulation of the waveform.
It is a further object of the present invention to provide a novel system and method to perform the detection and classification of a transmitted waveform with a minimum of calculations.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which it pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.